1. Field of the Invention
The present invention relates to a pneumatic radial tire capable of reducing not only a road noise in a high frequency band but also a road noise in a low frequency band.
2. Prior Art
Conventionally, in order to improve high speed endurance, a band layer in which band cords made of organic fiber such as nylon is spirally wound is provided around an outer side of a belt layer of a pneumatic radial tire. It is known that if such a structure is employed, resonance frequency of the tire is increased, and a road noise is reduced in high frequency band of 250 Hz or higher.
The present assignee has proposed a pneumatic tire capable of reducing the road noise without deteriorating passing noise by using higher modulus cords as the band cords and by limiting the characteristics of the band layer in a tread central region. For example, see Japanese Patent Applications Laid-open No. 2002-137606 and No. 2002-144814.
In a tire having a conventional band layer, however, if the road noise in the high frequency band is reduced, there is a problem that it becomes difficult to reduce a road noise in a low frequency band of about 125Hz to 160Hz, and the noise is increased depending upon frequency characteristics of a vehicle.
Thereupon, the present inventor researched a winding manner of the band cords. As a result, the present inventor found that if high modulus cords were wound loosely such that they were not extended so much instead of winding the high modulus cords such that a binding force was increased as in the conventional technique, not only a road noise in the high frequency band but also a road noise in the low frequency band could be reduced, and the inventor completed this invention.